The Preparation, Characterization, and Magnetism of Copper 15-Metallacrown-5 Lanthanide Complexes

The preparation and characterization of a series of encapsulated-lanthanide 15-metallacrown-5 complexes are
reported. Planar ligands such as picoline hydroxamic acid (picha) or nonplanar α-amino hydroxamic acids (e.g.,
glycine hydroxamic acid (glyha)) led to one-step syntheses of metallacrowns in yields as high as 85%. The reaction
of the appropriate hydroxamic acid with copper acetate and 1/5 equiv of gadolinium(III) or europium(III) nitrates
in DMF or water yielded crystals of Gd(NO3)3[15-MCCu(II)N(picha)-5], 1, Eu(NO3)3[15-MCCu(II)N(picha)-5], 2, and
Eu(NO3)3[15-MCCu(II)N(glyha)-5], 3. Several other 15-metallacrown-5 complexes were synthesized with (1) Cu(II)
or Ni(II) in the metallacrown ring metal position, (2) various lanthanides (La(III), Nd(III), Sm(III), Eu(III), Gd(III), Dy(III), Ho(III), Er(III), and Yb(III)) encapsulated in the center of the ring, and (3) chiral α-amino hydroxamic
acids (e.g., phenylalanine hydroxamic acid (H2pheha), leucine hydroxamic acid (H2leuha), and tyrosine hydroxamic
acid (H2tyrha)). It is believed that all of the complexes containing Cu(II) ions have the ring metals either in
four-coordinate, square-planar environments, bound to two tetradentate hydroximate ligands, or in five-coordinate,
square-pyramidal geometries if solvent is bound. Spectroscopic and magnetic characterization of the Ni(II)
complexes suggests that they are either five- or six-coordinate. The encapsulated lanthanides are generally pentagonal
bipyramidal, with five oxygen donors from the metallacrown ring and solvent or bidentate nitrate ions in the
axial positions. The circular arrangement of ions results in interesting magnetic behavior. With Dy(III) encapsulated
in the center of the ring, a magnetic moment as high as 10.9 μB is achieved. Analysis of the variable-temperature
susceptibility of La(NO3)3[15-MCCu(II)N(picha)-5] indicates that the five Cu(II) ions are antiferromagnetically coupled,
forming an S = 1/2 ground spin state with a moment of 1.7 μB at liquid helium temperatures. Complex 1 shows
ferromagnetic coupling of the Gd(III) ion to the five Cu ions at temperatures below 15 K. Studies of the
metallacrown complexes in solution show that they are stable and soluble in DMF and water. A proton relaxation
study on complex 1 has revealed a relaxivity of 9.8 mM-1 s-1 (20 °C and 30 MHz), a value that is comparable
to those of clinically useful MRI contrast enhancement agents. Complex 1 crystallizes in the triclinic space group
P1̄, with a = 12.657(3) Å, b = 14.833(3) Å, c = 17.707(3) Å, α = 79.65(2)°, β = 86.06(2)°, γ = 68.69(2)°, V
= 3046.6(12) Å, and Z = 2 (R1 = 0.0534, wR2 = 0.1289). Complex 2 crystallizes in the monoclinic space group
P21/n, with a = 16.319(2) Å, b = 21.863(2) Å, c = 18.410(3) Å, β = 96.85(1)°, V = 6522(2) Å3, and Z = 4 (R1
= 0.0463, wR2 = 0.0750). Complex 3 crystallizes in the triclinic space group P1̄, with a = 11. 173(6) Å, b =
11.534(6) Å, c = 13.311(5) Å, α = 93.81(3)°, β = 94.82(4)°, γ = 107.20(4)°, V = 1625(2) Å3, and Z = 2 (R1
= 0.1230, wR2 = 0.2979).